This invention generally relates to ink jet printing methods and more particularly relates to a method of compensating for malperforming or inoperative ink nozzles in a multitone ink jet printhead, so that high quality images are printed although some ink nozzles are malperforming or inoperative.
An ink jet printer produces images on a receiver by ejecting ink droplets onto the receiver in an imagewise fashion. The advantages of non-impact, low-noise, low energy use, and low cost operation in addition to the capability of the printer to print on plain paper are largely responsible for the wide acceptance of ink jet printers in the marketplace.
It is known that high quality printing by an ink jet printer requires repeated ejection of ink droplets from ink nozzles in the printer""s printhead. However, some of these ink nozzles may malperform, and may eject droplets that do not have the desired characteristics. For example, some malperforming nozzles may eject ink droplets that have an incorrect volume, causing the dots produced on the page to be of an incorrect size. Other malperforming nozzles may eject drops with an improper velocity or trajectory, causing them to land at incorrect locations on the page. Also, some malperforming nozzles may completely fail to eject any ink droplets at all. When such malperforming nozzles are present, undesirable lines and artifacts will appear in the printed image, thereby degrading image quality.
Malperforming and inoperative nozzles may be caused, for example, by blockage of the ink nozzle due to coagulation of solid particles in the ink. Techniques for purging clogged ink nozzles are known. For example, U.S. Pat. No. 4,489,335 discloses a detector that detects nozzles which fail to eject ink droplets. A nozzle purging operation then occurs when the clogged ink nozzles are detected. As another example, U.S. Pat. No. 5,455,608 discloses a sequence of nozzle clearing procedures of increasing intensity until the nozzles no longer fail to eject ink droplets. Similar nozzle clearing techniques are disclosed in U.S. Pat. No. 4,165,363 and U.S. Pat. No. 5,659,342.
Another reason for nozzle malperformance may be due to failures in electric drive circuitry which provides a signal that instructs the nozzle to eject a drop of ink. Also, mechanical failures in the nozzle can cause it to malperform, such as failure of the resistive heating element in thermal inkjet printer nozzles. Nozzle clearing techniques as described above cannot repair failed resistive heaters or failed electric driver circuits which may cause nozzles to permanently malperform. Of the course, presence of such permanently malperforming or inoperative nozzles compromises image quality.
European Patent Application EP 0855270A2 by Paulsen et al discloses a method of printing with an inkjet printhead even though some of the nozzles have failed permanently. As understood, this method provides for disabling portions, or xe2x80x9czonesxe2x80x9d, of the printhead that contain failed nozzles, and printing with the remaining zones containing functional nozzles. However, this method is has a draw back in that if all zones contain a failed nozzle, then correction is not possible. Also, the presence of any failed nozzles will increase the printing time considerably.
Other methods of compensating for malperforming nozzles are known that utilize multiple print passes. The concept of using multiple print passes to improve image quality is disclosed in U.S. Pat. No. 4,967,203 to Doan et al. In this method, which is referenced for its teachings, the image is printed using two interlaced print passes, where a subset of the image pixels are printed on a first pass of the printhead, and the remaining pixels are filled in on the second pass of the printhead. The subset of pixels is defined such that the pixels are spatially dispersed. This allows time for the ink to dry before the remaining pixels are filled in on the second pass, thereby improving image quality. Printing images using multiple print passes has another benefit in that for each nozzle there is at least one other nozzle that is capable of printing along the same path during the next (or previous) pass. This is used advantageously by Wen et al in the above cross referenced patent application, which discloses a method for compensating for failed or malperforming nozzles in a multipass print mode by assigning the printing function of a malperforming nozzle to a functional nozzle which prints along substantially the same path as the malperforming nozzle. This is possible when the functional nozzle is otherwise inactive over the pixels where the malperforming nozzle was supposed to print. However, this technique does not apply when it is required that ink be printed at a given pixel by more than one nozzle. In high quality inkjet systems, this is often desirable, as described hereinbelow.
To further improve image quality, modern inkjet printers provide for new ways of placing ink on the page. For example, several drops of ink may be deposited at a given pixel, as opposed to a single drop. Additionally, the plurality of ink drops placed at a given pixel may have different drop volumes and/or densities. Examples of these high quality inkjet systems are disclosed in U.S. Pat. Nos. 4,560,997 and 4,959,659. Each particular way that ink can be placed at a given pixel by one pass of a nozzle is called a xe2x80x9cstatexe2x80x9d. Different states may be created by varying the volume and/or density of the ink drop. The reason that this is done is that increasing the number of states in an inkjet printer increases the number of density levels that can be used to reproduce an image, which increases the image quality. For example, consider a binary inkjet printer that can place at each pixel either no drop or a single large (L) drop of fixed volume and density during a single print pass. This printer has only two states (per color), denoted as: (0) and (L). Correspondingly, this binary printer has only 2 fundamental density levels, and the intermediate densities are achieved by halftoning between the two available states. Now consider a modern inkjet printer that can print either no drop, a small drop (S), or a large drop (L) of a fixed density. This modern printer has three states: (0), (S), and (L). Taking this one step further; if the modern inkjet printer prints in a 2 pass interlaced mode, as discussed earlier, then two states can be placed at any given pixel. The number of fundamental density levels will be equal to the number of combinations of the available states (3) into groups of 2 (one state printed on each pass). In this case, the number of fundamental density levels will be six: (0,0), (0,S), (S,S), (0,L), (S,L), and (L,L). The intermediate densities are again created by halftoning between the available density levels, but as someone skilled in the art will know, the more density levels there are to render an image, the better the image quality will be.
To produce some of the fundamental density levels, more than one nozzle must be activated for a given pixel location during the printing process. For example, in a two pass interlaced print mode, printing a state of (S,L) at a given pixel location on the page requires that both of the nozzles that pass over the pixel are activated. This violates the constraints of the above discussed methods for correcting for malperforming nozzles. Thus, a different method of correcting for malperforming nozzles is required to achieve improved image quality on modem inkjet printers.
In a multiple pass print mode, one line of image pixels along the fast scan direction is printed by a group of ink nozzles with each ink nozzle printing that particular line of image pixels in each printing pass. If one of the ink nozzles in the group is malperforming (or inoperative), the printing job originally assigned to the malperforming nozzles can be assigned to a functional ink nozzle in that nozzle group, as described above. One shortcoming of this technique of correcting failed nozzles is that it does not adequately address all the possible situations of ink drop states. For example, in the above mentioned example, six density levels are produced by six sets of ink drop states: (0,0), (0,S), (S,S), (0,L), (S,L), and (L,L). The ink drop states (S,S), (S,L), and (L,L) do not have a (0) state within each of the ink state set. To use the above described correction method for malperforming nozzles requires abandoning at least one of the ink drop states in each of the ink drop sets; the abandoned ink drop state corresponding to the malperforming ink nozzle. The loss of one (or more) ink drop states will often significantly decrease the optical density below the intended density values. Although better than no compensation, this method for correcting malperforming nozzles still cannot completely eliminate image artifacts. Visible banding still exists on the printed image even if the digital image file is processed for this correction.
An object of the present invention is to provide a method of compensating for malperforming and inoperative ink nozzles in a multitone inkjet printer, so that high quality images are printed although some ink nozzles are malperforming or inoperative. With this object in view, the present invention provides for a method of compensating for at least one malperforming nozzle in an inkjet printing device having a printhead with a plurality of nozzles which are organized in nozzle groups, each nozzle group including a first nozzle which prints along a first row of image pixels, and at least a second nozzle which is capable of printing along substantially the same row of image pixels as the first row of image pixels, said nozzles adapted to printing an optical density at the image pixels using two or more states on a receiver in responsive to a swath data signal, wherein each state corresponds to a volume of ink that is desired to be emitted by a nozzle and a zero state corresponds to no ejection of an ink drop, comprising the steps of:
a) relating each optical density at an image pixel to a plurality of sets of states, and said sets of states being sequenced by the number of zero states in each set;
b) assigning a set of states to the image pixel wherein the number of zero states is at least equal to the number of malperforming nozzles in the nozzle group;
c) receiving the swath data signal and assigning a zero state in a set of states corresponding to a optical density on the receiver to each malperforming nozzle in the nozzle group, thereby producing a modified swath data signal; and,
d) printing the image pixels according to the modified swath data signal.
An advantage of the present invention is that high quality images are printed although some of the ink nozzles are malperforming or inoperative.
Another advantage of the present invention is that the malperforming or inoperative ink nozzles can be effectively compensated without substantial loss of density in the set of the ink drop states for each image pixel.
A feature of the present invention is that the malperforming or inoperative ink nozzles can be compensated for the set of ink drop states wherein none of the ink drop state is a zero state.
A further advantage of the present invention is that lifetime of the printhead is increased and therefore printing costs are reduced.
These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention.